1. Field of the Invention
This invention relates generally to subsea seismic exploration cables. More particularly, this invention relates to connection mechanisms between a seismic sensor phone and a subsea seismic exploration cable.
2. Description of the Related Art
The technique of seismically mapping a subsea formation has long been known in the field of oil and gas exploration. Traditional techniques for seismic mapping employ a number of ocean bottom cables ("OBC") laid parallel on the sea floor. Each cable is deployed from a recording boat via a tension controlled sheave and may be very long, sometimes in excess of 35,000 meters in length. A number of sensors, typically hydrophones, are connected to each cable at spaced intervals. A typical spacing interval for adjacent sensors is about 50 meters so the number of sensors for a given cable may be quite large, in excess of 5000 for a typical 35,000 meter OBC. OBC's typically consist of a plurality of individually insulated conductor wires disposed within a water resistant insulating jacket. Connection between a given sensor and the OBC involves penetrating the insulating jacket of the OBC and selectively removing one or more of the conductor wires for eventual splicing to the sensor. A pigtail wire is then connected to the splice ends of the OBC conductor wires and taped or tie-wrapped to the OBC. The sensor itself has a second pigtail that is connected to the first pigtail and may also be taped and tie-wrapped to the OBC. In some conventional OBC systems, the connection between the first pigtail and the splice ends of the OBC conductor wires is shrouded in a plastic housing. The combination of the plurality of OBC's and their respective plurality of sensors forms a seismic sensor array that is in communication with the recording boat. After the array is in place, acoustic pulses are passed into the subsea formation by air guns or other means. The behavior of the acoustic pulses is sensed and transmitted back to the recording boat by the array.
OBC systems must operate under harsh environmental conditions. During employment from and retrieval to the recording boat, the sensor drops are routinely subjected to rigorous forces from the rotating sheave mechanism that may nick, cut, or even sever the various pigtails as they pass across the sheave mechanism. The plurality of dangling pigtails presents a less than streamlined cross-section that must pass through the ship mechanism. In addition, the pigtails may hang up or be otherwise damaged by coral or other sea floor formations. Any perforation in any one of the pigtails may allow sea water to penetrate not only that particular pigtail, but also the associated OBC and possibly the entire cable array, rendering the entire array unuseable. As a result, the entire array must be recovered from the sea floor and either repaired or replaced with a new array.
There are a number of disadvantages associated with conventional sensor drops. The hundreds of individual pigtails associated with a single OBC represent literally hundreds of dangling cables that may be subject to perforation due to encounters with sea floor formations or during deployment from and retrieval to the recording boat. Furthermore, field repair of conventional sensor drops normally requires detailed procedures as well as special mixing of special potting materials. Specially trained personnel are ordinarily required to effect such repairs. In addition, structural failure of a pigtail while the OBC is submerged may result in total irretrievable loss of the sensor.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.